1. Field of the Invention
This invention relates to a method for processing an energy subtraction image such that noise occurring in the energy subtraction image, which is obtained from energy subtraction processing of radiation images, can be reduced.
2. Description of the Prior Art
Techniques for carrying out subtraction processing on radiation images have heretofore been known. When subtraction processing is to be carried out, a plurality of (basically, two) radiation images recorded under different conditions are photoelectrically read out, and digital image signals which represent the radiation images are thereby obtained. The image signal components of the digital image signals, which represent corresponding picture elements in the radiation images, are then subtracted from each other, and a difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images. With the subtraction processing method, the plurality of digital image signals are subtracted from each other in order to obtain a difference signal, and the radiation image of a specific structure can be reproduced from the difference signal.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order to extract the image of a specific structure of an object from the image of the entire object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure of the object is enhanced by the injection of contrast media. In the latter method, an object is exposed to several kinds of radiation having different energy distributions. Alternatively, the energy distribution of the radiation carrying image information of an object, is changed after it has been irradiated onto one of a plurality of radiation image recording media, after which the radiation impinges upon the second radiation image recording medium. In this manner, a plurality of radiation images, in which different images of a specific structure of the object are embedded, are obtained. Thereafter, the image signals representing the plurality of radiation images are weighted appropriately, when necessary, and subjected to a subtraction process, and the image of the specific structure of the object is thereby extracted.
Also, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation, which carries image information of an object, such as a human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, when it is exposed to the stimulating rays, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed, and the processed image signal is then used during the reproduction of a visible image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
In the radiation image recording and reproducing systems wherein the stimulable phosphor sheets are utilized, the radiation images stored on the stimulable phosphor sheets are directly read out in the form of the electric image signals. Therefore, with the radiation image recording and reproducing systems, the subtraction processing described above can be carried out easily. Also, in cases where radiation images are recorded on sheets of X-ray photographic film, the radiation images can be read out from the X-ray photographic film by using a microphotometer, or the like, and image signals representing the radiation images can thereby be obtained. Therefore, in such cases, it is possible to carry out the subtraction processing described above.
However, problems have heretofore been encountered in that an energy subtraction image, which is obtained from the energy subtraction processing, contains much noise at a low image density portion, to which only a small amount of radiation could reach during the image recording operation. In cases where a graininess improving process is carried out on the entire area of the energy subtraction image as in conventional techniques, high-frequency noise reduces in the low image density portion, but low-frequency noise remains in the low image density portion. As a result, an artifact giving a grainy feeling occurs in the image.
By way of example, in cases where the radiation image is the image of the chest of a human body, the low image density portion described above corresponds to a vertebral body, or the like. Therefore, adverse effects of the artifact on the actual diagnoses will be comparatively small. However, an energy subtraction image having such an artifact is very hard to see and gives an unfavorable impression to persons who see it.